The present invention relates to a method for operating a clutch, whose clutch slip is adjusted to a setpoint value, using a manipulated variable. The present invention also relates to a control device for a slip-controlled clutch, in order to implement the method.
A slip-controlled clutch can be used in the power train of a motor vehicle. Such a clutch usually transmits a torque applied to an input shaft, to an output shaft, via a number of frictionally interconnected coupling elements. In this context, the slip occurring between the coupling elements can result in the output shaft having a speed lower than that of the input shaft, an exact definition of the slip being the difference of the speeds of the input and output shafts.
The response characteristic and the general characteristic curve of such a clutch can be dependent on the current slip condition. This can be used in a clutch control system, in order to set a desired clutch performance. To this end, the clutch slip in a slip-controlled clutch is adjusted to a preselected, e.g. operating-point-dependent setpoint, by defining a manipulated parameter.
The object of the present invention is to specify a method for operating a clutch of the type mentioned above, which allows particularly reliable and stable control. In addition, the intention is to specify a control device that is particularly suitable for implementing the method.
The object of the present invention is achieved with regard to the method, in that a slip value is only selected as a reliable setpoint value, when the derivative of the friction coefficient/slip characteristic at this slip value exceeds a specifiable limiting value.
Advantageous refinements of the present invention are the subject matter of the dependent claims.
The present invention starts out from the consideration that, for particularly reliable and stable operation, the slip-controlled clutch should be operated, using a setpoint slip value, at which the clutch has self-stabilizing characteristics. In this context, self-stabilization of the clutch can be achieved using an operating point, at which the coefficient of friction of the coupling elements increases with increasing clutch slip. Using the increase in the friction coefficient, the clutch namely allows a comparatively large torque to be transmitted as a result of increasing slip. But if the transmitted torque remains constant with increasing slip, then the slip decreases automatically as a result of the increase in the coefficient of friction. In order to attain the self-stabilizing characteristics of the clutch, the operating point or setpoint value for the slip should therefore be selected in a range, in which the coefficient of friction continuously increases as a function of the slip.
In particular, the characteristic curve of the friction coefficient as a function of the slip can have a maximum as a result of the material properties of the coupling elements. In this case, it is intended that the setpoint value for the slip always be selected in an advantageous manner, to be less than the maximum point on the friction coefficient/slip characteristic. In other words, the setpoint value is advantageously selected as a function of a number of operating parameters, inside a range of permissible setpoint values; the slip value, at which the derivative of the friction coefficient/slip characteristic falls below a specifiable limiting value, being selected as an upper limit of the range of permissible setpoint values.
In order to detect a possible maximum in the friction coefficient/slip characteristic, it is provided that the relationship be monitored between a manipulated variable, by means of which the coefficient of friction is directly controlled and the slip is therefore indirectly controlled, and the slip. In this context, it is deduced that the coefficient of friction is continuously increasing as a function of the slip, and that the clutch therefore has the desired, self-stabilizing characteristic, when the magnitude of the derivative of the manipulated variable/slip characteristic does not yet fall below an additional, specifiable limiting value. In other words, the manipulated variable/slip characteristic is determined in order to evaluate the friction coefficient/slip characteristic. All of the slip values, for which the magnitude of the derivative of the manipulated variable/slip characteristic is greater than the additional limiting value, are considered to be permissible setpoint slip values in the sense of self-stabilizing performance.
The clutch is advantageously used in the power train of a motor vehicle. In an advantageous further refinement, the clutch is used on the output end of a continuously variable transmission. For such a continuously variable transmission is comparatively sensitive to torque surges, which may be introduced into the transmission, via the output shaft, due to changing road characteristics or driving situations. Therefore, the continuously variable transmission is advantageously protected against such torque surges, by inserting the clutch into the output shaft. In this context, the operation of the clutch in the self-stabilizing range is particularly advantageous for especially stable and reliable vehicle performance.
The method can be used in a control method for the clutch, in a particularly favorable and simple manner, by utilizing a parameter as a manipulated variable, which is simple to measure and physically available. To this end, a clutch pressure or a clamping force is advantageously used as a manipulated variable. In this context, the clutch pressure is particularly suitable for a method to operate a wet multi-plate clutch or a torque-converter lockup clutch, where the slip is set by the clutch pressure in each instance, the coefficient of friction being a function of the clutch pressure. In this case, the clutch pressure is therefore an available parameter already, and can be used in a particularly simple manner as an input variable for controlling the clutch. On the other hand, the clamping force is particularly suitable for a method to operate a dry clutch. In the case of such a clutch, the coupling elements are brought into frictional contact by an externally applied clamping force, the coefficient of friction between the coupling elements being a function of the clamping force. In this case, the clamping force is therefore an available parameter already.
For a wet multi-plate clutch, transmittable torque M is given as a function of coefficient of friction xcexc and clutch pressure P as follows:
M=xcexcxc2x7rxc2x7zxc2x7(Axc2x7Pxe2x88x92F), 
where r is the effective friction radius, z is the number of friction surfaces, A is the piston area of the friction-clutch actuating device, and F is a minimum force for power transmission. In the case of a constant transmitted torque M, this function yields the following relationship between clutch pressure P and coefficient of friction u:
xe2x80x83P=1/Axc2x7(M/(xcexcxc2x7rxc2x7z)+F).
Therefore, clutch pressure P decreases with increasing coefficient of friction u; thus, a maximum in the functional relationship between coefficient of friction xcexc and the slip corresponds to a minimum in the relationship between clutch pressure and slip. Therefore, a setpoint slip value less than the minimum point on the clutch pressure/slip characteristic is selected when using the clutch pressure as a manipulated variable. Even when the derivative of the clutch pressure/slip characteristic has a negative value in this range, one can infer that a setpoint slip value is permissible in this sense, when the derivative of the clutch pressure/slip characteristic does not yet exceed the specifiable limiting value.
For a dry clutch in which the clamping force is used as a manipulated variable, the same relationship regarding the functional dependence of slip is obtained from a qualitative standpoint: the clamping force also decreases with increasing coefficient of friction xcexc.
In order to continue ensuring reliable and especially verifiable operational performance, even after a comparatively long clutch running period, the setpoint value of the slip-controlled clutch is advantageously checked and, if necessary, updated during operation, in the form of adaptive correction, after expiration of predefined maintenance intervals, or also periodically. In this manner, e.g. aging effects or changes in the operational performance resulting from material fatigue can be compensated for.
To update the setpoint value, the setpoint value is purposefully varied in a phase in which the coupling torque to be transmitted is constant over time, and the resulting slip, i.e. the difference in the speeds of the input and output shafts, is measured. Using the manipulated variable/slip characteristic measured in this manner, it is then checked if the magnitude of its derivative exceeds the preselected limiting value, within the slip-parameter range regarded as the permissible setpoint-value range until now. If this is the case, then the upper limit of the range of permissible setpoint values is advantageously updated, using the preceding measurement.
Depending on the operating conditions, it can be desirable to operate the slip-controlled clutch with as high a slip as possible, e.g. in order to ensure an especially large, available control range. In order to also ensure self-stabilizing clutch performance in this case, the setpoint value is advantageously selected to be approximately equal to the upper limit of the range of permissible setpoint values.
With regard to the control device, the named objective is achieved, using a selector unit, which selects the setpoint value as a function of a number of operating parameters, inside a range of permissible setpoint values; and using a diagnostic unit, which only allows a setpoint value to be a permissible setpoint value, when the derivative of the friction coefficient/slip characteristic at this point exceeds a specifiable limiting value.
For a particularly simple and, therefore, reliable selection procedure, the control unit is designed in an advantageous refinements, in such a manner, that the selection unit selects the setpoint value inside a range of permissible setpoint values. As an upper limit of the range of permissible setpoint values, the diagnostic unit advantageously selects the slip value at which the derivative of the friction coefficient/slip characteristic falls below a specifiable limiting value.
The control device advantageously selects a clutch pressure or a clamping force as a manipulated variable. When the control device is designed for the selection of the clutch pressure as a manipulated variable, the control device is therefore particularly suited for use with a wet multi-plate clutch or a torque-converter lockup clutch. However, the control device is particularly suited for use with a dry clutch, when it is designed for the selection of clamping force as a manipulated variable.
In order to allow the permissible setpoint value range to be updated, and thus, allow possible aging effects to be compensated for, an advantageous further refinement provides for the diagnostic unit being connected on the input side to a measuring device for ascertaining the clutch slip.
In particular, the advantages attained by the present invention consist in a self-stabilizing performance of the clutch, and thus, an especially stable operational performance, being ensured by limiting the range of permissible setpoint slip values to a range, in which the friction coefficient/slip characteristic continuously increases. By selecting an upper limiting value for the slip in a range in which the friction coefficient/slip characteristic still has at least a minimum slope, the operating point of the clutch is also selected so that the self-stabilizing performance of the clutch is even maintained in the case of slight system deviations. Therefore, undesirable frictional vibrations, which can especially occur at an operating point at which the coefficient of friction decreases with increasing slip, can also be substantially ruled out for the operation of the clutch.